Two-Part Shroud or Shroud Section for a Stator Stage with Vanes of an Axial Compressor

ABSTRACT

A shroud or shroud section for a stator stage with vanes of an axial compressor is made of composite material and is built in two parts, each having a series of notches corresponding to a half of the profile of the vanes, so as to form apertures that follow the shape of the vanes when the two parts are assembled. The two parts also include mutual fastening members or means adapted to ensure their assembly.

This application claims priority under 35 U.S.C. §119 to European PatentApplication No. 09179128.5, filed 14 Dec. 2009, which is incorporatedherein by reference for all purposes.

BACKGROUND

1. Field of the Application

The present application relates to a shroud for a stator with vanes ofan axial compressor, more particularly to an internal shroud of astator, more particularly still, to an internal shroud made of compositematerial. Such compressors are typically present in turbojet engines,jet prop engines, and gas generators.

2. Description of Related Art

An axial compressor typically comprises a series of compression stages,each being constituted of a circumferential arrangement of vanes mountedon their bases on a rotor. A stator serving as a casing surrounds therotor and the blades. The ends of the rotor blades move in the vicinityof the internal surface of the stator. The fluid, typically air, is thusdisplaced and compressed according to an annular jet, concentric to therotation axis of the rotor. The rotor thus comprises several rows ofcircumferential blades, spaced apart from one another. Rows ofstationary vanes are mounted on the stator between the rows of rotorblades for the purpose of rectifying the airflow between two stages ofthe compressor. These stator vanes conventionally comprise an internalshroud at their ends for the purpose of delimiting the internal envelopeof the fluid stream. This shroud conventionally has an annular shapewith an outer surface that is shaped specifically for the defining ofthe flow. The outer surface comprises a series of apertures or holesadapted to receive the inner ends of the vanes which are fixed at theirouter ends or bases to the stator. This shroud also ensures that thevanes are affixed to one another in the area of their inner ends. Theinternal surface of the shroud is lined with a friable material or elsemore conventionally called “abradable” from the English term. This layerof abradable material is adapted to cooperate by friction with one orseveral circumferential ribs on the rotor in order to make it somewhatleak-proof. These ribs are more conventionally called knife edges.

The patent document EP 1 213 484 A1 of the same applicant as thisapplication discloses an internal shroud and an external shroud of acompressor stator stage, these shrouds being conventionally made ofmetal. This document relates to a device for connecting the vanes to theshrouds by means of a band which is inserted in apertures made at eachend of the vanes passing through the holes of the shrouds, the bandserving as a locking key with respect to the outer and inner surfaces ofthe external and internal shrouds, respectively.

As a function of the dimensions and materials used and also tofacilitate the assembly, it can be interesting to segment the shroudinto several sections. Indeed, in the case of a shroud made of compositematerial, for example, it can be difficult to inject the resin of thecomposite material over lengthy pieces, which means that it isinteresting to segment the shroud so as to reduce the length of thesections. Segmenting the shroud can also prove to be interesting tocompensate for the differential expansions in the area of the vanesthemselves and of parts of the stator supporting the vanes. Theaerodynamic performance of a segmented composite shroud is diminishedfor the following reasons.

The method of manufacture by plastic or resin injection requires the useof a mold to give the final shape. The general shape of a section is anarc of circle whereas the unmolding direction on a surface of thesection is axial and not radial. The shroud holes have main axes whichcorrespond to radii starting from the rotation axis of the rotor; theseaxes are therefore inclined with respect to the unmolding direction.Consequently, the holes have an unnecessarily flared section toward theunmolding direction. The adjustment between the holes of the shroud andthe vanes is unnecessarily large and unfavorable from an aerodynamicstandpoint. It is indeed desirable for the shroud surface in contactwith the fluid stream, particularly in the area of the junction with thevanes, to be as continuous as possible.

Furthermore, independent of this requirement related to the unmolding,the holes of a segmented composite shroud must be provided to beslightly larger than the section of the vanes which go through thembecause of the assembly process. Indeed, once a first section is inplace, it needs to be pulled up along the vanes in order to leaveminimum room for the adjacent end of the section to be set in placenext, so the first section can then be pulled down toward its finalposition. The clearance increase between the holes and the vanes, inparticular in the area of the outer surface, is detrimental to theaerodynamics of the stator stage.

Although great strides have been made in the area of axial compressors,many shortcomings remain.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial, cross-sectional view of an axial compressor with,among other things, a shroud according to the present application.

FIG. 2 is a perspective view of a shroud section according to thepresent application.

FIG. 3 is a perspective view according to another angle of the shroudsection according to FIG. 1 mounted on a row of vanes.

FIG. 4 is a bottom, perspective view of the shroud section of FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present application discloses a shroud or shroud section made ofcomposite material for a stator stage with vanes of an axial compressor,the shroud or shroud section being adapted to receive an end of thevanes, comprising at least one first curved element with at least onefirst notch corresponding to a first portion of the contour of the endof the vanes, at least one second curved element having a generallysimilar shape as the first curved element with at least a second notchcorresponding to a second portion of the contour of the end of thevanes, the first and second notch or notches forming one or severalpair(s) of notches, the first and second curved elements being adaptedto be arranged axially against one another so as to confine the profileof the end of the vanes in each pair of first and second notches. Thesecond portion of the contour of the end of the vanes preferablycorresponds to the rest of the contour which is not concerned with thefirst portion, so that the first and second notches surround theentirety of the contour of the vanes. Such a construction has numerousadvantages, mostly from the standpoint of an ease of assembly on thevanes and from the standpoint of aerodynamics of the flow of the fluidstream into the stator.

Indeed, this modular construction with the pairs of notches forming theapertures of the shroud enables a substantially easier assembly incomparison with a conventional segmented shroud, each section of whichbeing required to be carefully placed in front of the series of vanes towhich it must be mounted and must then be pulled up along the vanes inthe area of one lateral end in order to leave enough room for theadjacent end of the next section.

Furthermore, this modular construction makes it possible to use anotherway of unmolding the segments made of composite material by getting ridof the unavoidable requirement with the segments of a conventionalshroud of having to unmold in a direction forming an angle with the maindirections of some apertures thus formed. The apertures of each segmentof a conventional shroud are indeed different from one another in orderto take into account the projection of the vane according to thedirection of assembly. These differences lead to assembly adjustmentswhich are too great, having a detrimental effect on the aerodynamics ofthe flow.

Each curved element forming the shroud comprises at least two,preferably three, even more preferably at least four, notches.

According to an advantageous embodiment of the present application, eachof the first and second curved elements comprises an assembly edge in aplane, perpendicular to the main axis of the compressor, the assemblyedges being adapted to come into contact with one another during theassembly of the two curved elements, preferably, by becoming closertogether in a direction corresponding approximately to the flowdirection.

According to another advantageous embodiment of the present application,the first and/or second portion of the contour of the end of the vanescorrespond(s) to approximately half of the profile in the direction ofthe flow. This measure allows the apertures to take on the shape of thebarrel-shaped vanes.

According to yet another advantageous embodiment of the presentapplication, the first curved element comprises an assembly edge adaptedto cooperate by insertion with an assembly edge of the second segmentduring the assembly of the two curved elements, preferably by becomingcloser in a direction corresponding approximately to the direction ofthe flow.

According to yet another advantageous embodiment of the presentapplication, the assembly edge of one of the first and second curvedelements comprises a groove according to the curved direction and theassembly edge of the other of the first and second curved elementscomprises a projecting portion, the groove and the projecting portionbeing interrupted by the respective notch or notches of the first andsecond curved elements.

According to yet another advantageous embodiment of the presentapplication, the shroud is an internal shroud with a U-shaped sectionwith the opening of the U-shape being directed toward the center of theshroud so as to form a recess adapted to receive some abradable materialand where the limit between the first and second curved elements islocated approximately toward the center of the U-shape.

According to another advantageous embodiment of the present application,each of the first and second curved elements comprises member or meansfor mutual fastening, these members or means being preferably of theclip-type.

According to another advantageous embodiment of the present application,the mutual fastening means or members are such that they cooperate byelastic engagement of surfaces with positive contact when the first andsecond curved elements move closer together in a direction correspondingapproximately to the flow direction.

According to another advantageous embodiment of the present application,the mutual fastening means or members comprise several elementsprojecting over one of the first and second curved elements in adirection corresponding approximately to the direction for moving thefirst and second curved elements closer together.

According to another advantageous embodiment of the present application,the mutual fastening means or members are arranged on the respectiveassembly edges of the first and second curved elements.

According to another advantageous embodiment of the present application,each end of the shroud section comprises a section whose shapecorresponds to that of the end of the vanes so that the junction betweentwo adjacent sections corresponds to the shape of the vane.

According to another advantageous embodiment of the present application,each end of the shroud section comprises means for connecting to anadjacent section, these connection means having a positive contact,preferably on both sides of the junction on each of the first and secondcurved elements.

According to another advantageous embodiment of the present application,the first and second curved elements are each ring-shaped, preferablyclosed.

According to another advantageous embodiment of the present application,one of the first and second curved elements is ring-shaped, preferablyclosed, and comprises several second curved elements corresponding tosections of the ring and adapted to be arranged successively against thering-shaped curved element so as to confine, over the entire perimeterof the ring, the shape of the vane roots in each pair of first andsecond notches.

The present application also comprises a shroud for a stator stage withvanes of an axial compressor comprising several shroud sections such asdescribed hereinabove.

FIG. 1 shows a cross-sectional view of part of an axial compressor,typically of a jet engine or turbine engine. The compressor comprises arotor 5 rotating about the axis 2. The rotor comprises a series ofblades 6 fixed onto its circumference, corresponding to a stator stage.The flow direction of the fluid to be compressed is shown by the arrow.The casing 3, or the stator, of the compressor comprises a series ofvanes 4 fixed at their bases. This series of vanes constitutes a statorof the fluid stream located between two blade rows of the mobile wheel,the upstream row not being represented. A row of blades of the mobilewheel and the guide vane downstream constitute a stage of thecompressor. The inner ends of the vanes 4 are connected to a shroud 8.The shroud has the general shape of a circular ring following the shapeof the rotor 5. The shroud 8 thus delimits the lower or internal part ofthe annular fluid stream passing through the stator. It is kept in placeby the vanes and makes the rotor 5 leak-proof. The shroud comprises onits inner surface a layer of abradable material 10. The rotor 5comprises two circumferential ribs, more commonly called knife edges 9,cooperating with the layer of friable material 10. The abradablematerial 10 is applied and then machined so as to present a sealingsurface 11 cooperating with the knife edges. This material has frictionproperties with the metal of the knife edges and the ability todisintegrate into fine dust in case of contact with the knife edges whenthe rotor is in rotation. The knife edges 9 and the surface 11 made offriable material thus constitute a labyrinth seal.

The shroud 8 is constituted of a series of sections shaped in anarc-of-circle such as shown in FIG. 2. The section shown in FIG. 2 isconstituted of two elements in an arc-of-circle 12 and 14, each formingabout a half, in a direction corresponding to that of the axis of themachine, of the shroud section along to a median arc-of-circle.

The curved element 12 comprises a series of notches 16 adapted to followthe shape of the corresponding end of the vanes. The notches 16 are, inprinciple, identical, taking into account the fact that all the vanes ofa stator stage are generally identical and uniformly distributed on thecircumference of the shroud. The notches 16 are such that theycorrespond to about half of the profile of the vanes, the half being ina general direction corresponding to the flow direction of the fluid, orto the longitudinal direction of the section of a vane. The curvedelement 12 comprises an outer surface 22 of revolution, correspondinggenerally to a segment or piece of a cylinder, that is, a surfacedirected toward the outside of the circle or ring which forms thecomplete shroud. This surface is the surface delimiting the innerenvelope of the fluid stream passing through the stator. It is delimitedon one side by a rounded edge 26 and on the other side by an edge 27crossing through the notches 16 and adapted to come into contact with acorresponding edge 29 of the other curved element 14. This contact edge27 comprises a groove 28 extending in parallel to the outer surface 22and below this surface. The groove 28, as the edge 27, is interrupted bythe notches 16.

The curved element 14 opposite the curved element 12 is generallysimilar. It comprises a series of notches 18 corresponding to thenotches 16 of the opposite curved element 12 and adapted to form, whenthe two curved elements are arranged in an adjacent and correspondingmanner, a series of apertures or openings which follow the shape of thevanes. The shape of the notches 18 thus corresponds to the rest of thesection of the vanes, that is, to the portion of the contour of the vanewhich is not covered by the notch 16 of the opposite curved element 12.As a result, the notches 18 can have a slightly or even substantiallydifferent profile than those of the opposite notches 16, as a functionof the section of the end of the vane to be surrounded. Similar to theopposite curved element 12, the curved element 14 comprises an outersurface 22, generally cylindrical or ring-shaped, that is, a surfacedirected toward the outside of the circle or ring which forms thecomplete shroud. This surface is the surface delimiting the innerenvelope of the fluid stream passing through the stator. It is delimitedon one side by a rounded edge 24 and on the other side by an edge 29passing through the notches 18 and adapted to come into contact with thecorresponding edge 27 of the opposite curved element 12. This contactedge 29 comprises a tongue 30 or lug extending in parallel to the outersurface 22 and at a lower level of this surface. The tongue 30, as theedge 29, is interrupted by the notches 18.

The curved element 14 also comprises in the area of its contact edge 29,at a level that is lower than that of the outer surface 22, a series ofelastic hooks generally directed toward the opposite curved element 12during the assembly and adapted to cooperate by flexion followed by amovement for returning to a position close to the initial position withcorresponding housings provided in the area of the contact edge 27 ofthe opposite curved element 12. These hooks are arranged by pairs with agenerally U-shaped profile directed toward the outside and where theends of the U-shaped legs each comprise a boss forming the hook as suchwith a surface inclined with respect to the direction for inserting thehooks in their housings 21 of the opposite curved elements and with asurface that is generally perpendicular to this direction and being ableto come into positive contact with a corresponding surface of thehousings. The bosses in question are arranged at each end of the twoU-shaped legs and so as to be laterally directed toward the outside ofthe U-shape.

The shroud section is provided to be mounted by assembling the twoelements about the vanes. In practice, the two elements 12 and 14 arearranged in front of one another, similar to the drawing of FIG. 2, byfirst engaging each of the elements with the row of vanes which isadapted to be fixed to this shroud section. Once each element is engagedwith the row of vanes, that is when the notches of each elementcooperate, at least partially, with the vanes, moving the two elementscloser together is very easy since it is guided, at least in a planegenerally at a tangent to the outer surface 22 of the shroud, by thevanes. Putting the respective contact edges 27 and 29 in correspondencecan require one or more movements in a direction that is perpendicularto the plane at a tangent to the outer surface 22 of the shroud. Thetongue 30 of the element 14 is arranged facing the groove 28 of theopposite element 12 so as to ensure an efficient guiding during theassembly. The mutual fastening of the two elements is carried out byapplying a force for moving the contact edges closer together so as tomake the resilient hooks penetrate in the respective housings up to thepoint where the parts with the positive contact surfaces override thecorresponding surfaces of the housings. The legs can then return to aposition close to that before insertion and where the positive contactsurfaces of the hooks mesh with the corresponding surfaces of thehousings.

Alternatively, the shroud section can be assembled before the vanes aremounted. The elements 12 and 14 are thus assembled by a similar movementto that described hereinabove, but without the presence of the vanes.The section is then mounted pre-assembled on the vanes. This principleof assembly, just like the previous assembly principle described indetail in the previous paragraph, provides the advantage of allowing ashroud with apertures and a profile that is very adjusted to the profileof the vanes to be made, while doing away with the requirements causedby the unmolding of a conventional shroud section at the apertures. Theunmolding of a conventional shroud section is generally done by movingaway part of the mold corresponding to the outer surface and to theapertures over at least part of the thickness of the shroud from theouter surface. The curved nature of the shroud section causes the axesof the apertures to correspond to radii of the shroud and can thus forman angle with the unmolding direction. This requirement calls for asection of the apertures that flares more than what is required for theassembly. However, the previous principle of assembly described indetail in the previous paragraph has the added advantage to make iteasier to assemble the shroud and mount it on the vanes.

FIG. 3 shows the shroud section of FIG. 2 once assembled and mounted ona row of vanes 4. One can see that the outer surface 22 of the shroud 8remains continuous and regular in the junction area. The mutualfastening is ensured by a pair of elastic hooks 20 between each pair ofneighboring vanes. The vanes 4 can be fixed to the shroud by applyingglue or an elastomer serving as glue ensuring a dual function, namely,that of ensuring a mechanical connection and a tight sealing. To do so,the ends of the vanes slightly project under the apertures so as toreceive the glue or the elastomer.

It must be noted that other methods for mechanically connecting thevanes and the shroud can be provided. Indeed, it is absolutelyconceivable to provide for a band to be placed through the holes made inthe ends of the vanes so as to ensure a mechanical connection preventingthe vanes from exiting the shroud, such as disclosed in the document EP1213484 A1. In this case, it would be more practical to provide for aband to be placed before the shroud is mounted on the vanes.

FIG. 3 also shows the U-shaped section with the open portion directedtoward the center of the shroud, forming a recess adapted to receive theabradable material in the form of a paste by projection or by any otherequivalent or similar method.

FIG. 4 is an enlarged view of a part of the bottom of the shroud sectionmounted on the vanes shown in FIG. 3. It shows well the precision of theadjustment between the apertures formed by the pairs of notches 16 and18 and the vanes 4. The fact that each notch of a pair of notches 16 and18 forming an aperture surrounds the blade over approximately half ofits length (according to the flow direction) makes it possible toprecisely adjust the apertures on tunnel-section vanes, that is, vaneswhose section is substantially thicker in the middle than on the edges(leading or trailing).

The shroud sections such as shown in FIGS. 2, 3, and 4 can have endswith a profile corresponding to that of the end of the vanes, so thatthe junction between two adjacent sections corresponds to the profile ofthe blade (not shown). Preferably, each shroud section end comprisesmeans for connecting to an adjacent section, these connection meansbeing of the positive contact type, preferably on both sides of thejunction on each of the curved elements forming a section. This positivecontact ideally cooperates with a relative sliding movement along vanesbetween two adjacent sections.

It must be noted that the shape, number, and position of the elastichooks as mutual fastening means for curved elements are given only byway of non-limiting example. Indeed, various alternatives which aresimilar and/or equivalent to these fastening means are possible, as afunction of diverse parameters such as the choice of materials,manufacturing requirements (molding/unmolding for non-metallicmaterials), ease of assembly and disassembly, desired resistance todisassembly.

It must also be noted that the shroud can be made of two elements of thetype previously described, namely the curved elements 12 and 14, eachforming a complete ring. These elements will preferably form a closedring. Alternatively, the shroud can be composed of a first elementforming a complete ring and preferably closed, and a series of curvedelements adapted to be arranged against the first element and adjacentto one another along the circumference of the first element.

It must also be noted that it is, of course, conceivable to apply theshroud according to the present application to a stator stage whosevanes are not necessarily identical and/or uniformly distributed overthe entire circumference. In this case, the apertures formed by thepairs of notches will not be uniformly distributed but rather accordingto the vanes of the stator.

It must also be noted that, although the embodiment of the presentapplication was described in relation to an internal shroud, the presentapplication is similarly applicable to an external shroud.

The present application discloses a shroud that overcomes theaforementioned drawbacks and has many benefits and advantages, includinga shroud that is easy to mount and has good aerodynamic characteristics.

It is apparent that an invention with significant advantages has beendescribed and illustrated. The particular embodiments disclosed aboveare illustrative only, as the invention may be modified and practiced indifferent but equivalent manners apparent to those skilled in the arthaving the benefit of the teachings herein. It is therefore evident thatthe particular embodiments disclosed above may be altered or modified,and all such variations are considered within the scope and spirit ofthe invention. Accordingly, the protection sought herein is as set forthin the description. Although the present application is shown in alimited number of forms, it is not limited to just these forms, but isamenable to various changes and modifications without departing from thespirit thereof.

1. A shroud for a stator stage with vanes of an axial compressor,comprising: at least one first curved element having at least one firstnotch, each first notch corresponding to a first portion of a contour ofan end of one of the vanes; and at least one second curved elementhaving at least one second notch, each second notch corresponding to asecond portion of the contour of the end of the vane, the first andsecond curved elements being adapted to be arranged axially against oneanother, so as to confine the ends of the vanes in corresponding pairsof first and second notches.
 2. The shroud according to claim 1, whereineach of the first and second curved elements comprises: an assembly edgein a plane that is perpendicular to a main axis of the compressor, theassembly edges being adapted to come into mutual contact during theassembly of the two curved elements by becoming closer in a directiongenerally corresponding to a flow direction through the compressor. 3.The shroud according to claim 1, wherein each first portion of thecontour of the end of a vane generally corresponds to half of theprofile of the end of such vane in a flow direction of the compressor.4. The shroud according to claim 1, further comprising: a first assemblyedge disposed on the first curved element; and a second assembly edgedisposed on the second curved element; wherein the first assembly edgecooperates by insertion with the second assembly edge during assembly ofthe first curved element and the second curved element by becomingcloser in a direction corresponding to a flow direction of thecompressor.
 5. The shroud according to claim 1, further comprising: agroove disposed along the curved direction of one of the first assemblyedge and second assembly edge; and a projecting part disposed along thecurved direction of the other of the first assembly edge and secondassembly edge; wherein the groove and the projecting part areinterrupted by the respective first and second notches of the first andsecond curved elements.
 6. The shroud according to claim 1, furthercomprising: an internal U-shaped section having an opening of theU-shape directed toward the center of the shroud, so as to form a recessadapted to received an abradable material.
 7. The shroud according toclaim 6, wherein the first and second curved elements intersect atapproximately the center of the U-shape.
 8. The shroud according toclaim 1, further comprising: clip-type fastening members disposed on thefirst and second assembly edges.
 9. The shroud according to claim 8,wherein the clip-type fastening members cooperate by elastic engagementof positive contact surfaces when the first and second curved elementsbecome closer together along a direction corresponding to a flowdirection of the compressor.
 10. The shroud according to claim 8,wherein the clip-type fastening members comprise: a plurality of spacedelements projecting beyond one of the first and second curved elementsalong a direction corresponding approximately to the direction formoving the first and second curved elements closer together.
 11. Theshroud according to claim 8, wherein the clip-type fastening members arearranged radially interior to the respective assembly edges of the firstand second curved elements.
 12. The shroud according to claim 1, whereinthe first curved element and the second curved element are each formedby a plurality of curved sections, the end of each section having aprofile that corresponds to the end of a vane, such that the junctionbetween adjacent curved sections corresponds to the vane profile. 13.The shroud according to claim 1, wherein at least one of the firstcurved element and the second curved element is formed by a plurality ofcurved sections, the end of each section having a profile thatcorresponds to a portion of the end of a vane, such that the junctionbetween adjacent curved sections corresponds to a portion of the vaneprofile.
 14. The shroud according to claim 13, wherein each end of eachcurved section includes a means for connecting to an adjacent section,each means for connecting including a positive contact on both sides ofthe junction on each of the first and second curved elements.
 15. Theshroud according to claim 1, wherein each of the first and second curvedelements is in the shape of a closed ring.
 16. The shroud according toclaim 1, wherein one of the first and second curved elements forms aclosed ring shape, and the other of the first and second curved elementsis formed by a plurality of curved sections corresponding to the closedring shape, each curved section being successively arranged against theclosed ring shape so as to confine, over the entire perimeter of theclosed ring shape, the roots of the vanes in corresponding pairs offirst and second notches.
 17. The shroud according to claim 1, whereinthe shroud forms a shroud section of the compressor.
 18. The shroudaccording to claim 1, wherein the shroud is made of a compositematerial.